1. Field of the Invention
The invention relates to cutting off a multiplicity of wafers from a workpiece with a wire saw, and in particular to the wire saw as well as to improved methods which can be carried out with the wire saw.
2. The Prior Art
Wire saws are used in particular to cut off a multiplicity of semiconductor wafers from a cylindrical crystal in one working step. A sawing wire, which is wound around two or more rotatably mounted wire-guide rollers, forms wire webs, of which at least one can be used to cut off wafers. The wire web is bounded by two wire-guide rollers, the longitudinal axes of the wire-guide rollers being oriented perpendicularly to the sawing wire in the wire webs. The wire-guide rollers of a wire saw are set in rotation by at least one drive means. During the cutting-off operation, a relative movement is produced between crystal and wire web by means of a feed device. During this cutting procedure, the crystal passes through the wire web. In the process, a suspension designated as "slurry" and containing cutting abrasive particles is fed to the wire web. However, the sawing wire may also be covered with firmly bonded cutting abrasive particles. On this basis, the suspended cutting abrasive particles can be dispensed with and, if need be, a liquid cooling lubricant can to be supplied.
The greatest precision possible is required to cut off semiconductor wafers from a crystal. A cut-off semiconductor wafer should have sides which are as flat and as parallel to one another as possible. Alternatively, the wafer can have another, particular shape. Deviations from the desired wafer shape result whenever the sawing wire in the wire web leaves a desired position during the cutting-off operation. This may be caused, for example, by movement of the feed device or movement of the crystal or movement of at least one wire-guide roller forming a boundary of the wire web. This will occur provided these movements have components which are directed in the longitudinal direction of the wire-guide rollers. Despite extensive measures for cooling the wire-guide rollers and their bearings, it may not be possible to completely avoid such axial movements of the wire-guide rollers. These axial movements are caused by thermal expansion of the wire-guide rollers as a result of overheating, which may result in an undesirable and incorrect positioning of the sawing wire in the wire web.
EP 733 429 discloses how undesirable axial movements of the wire-guide rollers or of the feed device can be measured and corrected. However, these proposals are not suitable for favorably influencing the axial thermal expansion of a wire-guide roller caused by overheating. This prior art problem is solved by the present invention.
The necessity for changing wire-guide rollers of known wire saws is complicated and involves lengthy down times during which the wire saw cannot be used. A reason is that the wire-guide rollers are very heavy and, therefore, are difficult to manipulate. The removal of a wire-guide roller is complicated, since it also requires the removal of a moveble bearing or other machine parts which are in the way. Also, this removal takes a great deal of time. An advantage of the present invention is that changing the wire-guide rollers is substantially simpler.